A professional extractor examines a sheet of marijuana concentrate known as "shatter" after removing it from the oven at Mahatma Concentrates in Denver on May 1, 2014. (Brennan Linsley, Associated Press file)

It is unclear which of the two companies — Mahatma Concentrates or Treatments Unlimited, both in Denver — was already under scrutiny, and officials with the Colorado Department of Agriculture would not elaborate.

But the agency said investigations into the businesses’ use of pesticides is the focus of its probe.

“The CDA has one ongoing investigation with one of the growers, and based on the information supplied in the article, we have opened a second investigation into the other named grower to determine if their pesticide use is in compliance with” the Colorado Pesticide Applicator’s Act, CDA spokeswoman Christi Lightcap said in an e-mail.

Penalties on the growers could range from a simple order to stop the prohibited conduct to fines of up to $1,000 for each violation. Under the act, repeat offenders’ fines are doubled and chronic offenders face potential license sanctions, Lightcap said.

Although laws require marijuana businesses to test for pesticide residues, that rule has not been enforced because only one facility is certified to do that analysis. Colorado Marijuana Enforcement Division rules show marijuana that tests positive for unapproved pesticides and other contaminants must be destroyed or decontaminated.

The Post on Tuesday reported that two of eight lab tests it commissioned on marijuana extracts found three unapproved pesticides in products made by Mahatma.

Although Mahatma labels indicated the tainted marijuana used to make the extracts came from its own grow facility, documents showed the cannabis actually came from Treatments. A Treatments co-owner admitted that the company had used products that contained the unapproved pesticides.

Treatments, which operates a pair of Altitude The Dispensary shops in Denver, was one of 11 marijuana growers to have about 100,000 plants quarantined earlier this year by Denver health officials over pesticide concerns. The city allowed most of the plants back into commerce, including those grown by Treatments, when levels of pesticide residues dropped to the lowest amount allowed on food crops. Two other businesses destroyed the plants rather than wait for pesticide test results.

Retailers with recalled products were instructed to remove them from store shelves and either destroy the products in compliance with MED regulations or return the products to the manufacturer, said Dan Rowland, spokesman for Denver’s office of marijuana policy.

Consumers should either destroy any recalled product they have or return it to the store of purchase, Rowland said.

“This information of potential pesticide misuse is concerning,” Lightcap said in announcing the investigations. “The CDA has identified a list of products that can be used on marijuana, and it has been available to growers for well over a year.”

Pesticides on marijuana is a controversial topic because no science exists to say which products are safe for consumers. Although several pesticides are approved for use on some fruit and vegetable crops, the U.S. Environmental Protection Agency cannot say whether they are safe on marijuana because the plant is illegal under federal law.

CDA has compiled a list of pesticides — some of them so nontoxic that they do not require federal registration — that can be used on marijuana based on their label restrictions, not on their safety.

It is a violation of federal law to use any pesticide contrary to its label directions. CDA has chosen those products whose label restrictions are so broad that using them on marijuana would not be a violation.

But CDA officials have consistently said they do not recommend using pesticides on marijuana, nor will they say those products that appear on its list of approved pesticides are safe to consumers.

“We are in a completely new business here, and we’re building a space shuttle that’s not been done anywhere else,” Colorado agriculture Commissioner Don Brown said.

Other states, including New Hampshire, have restricted pesticides allowed on marijuana only to those so nontoxic that federal registration is not required.

Colorado nearly did the same, but the industry argued that nontoxic pesticides — mainly organics such as neem, cinnamon and peppermint oils — weren’t nearly strong enough to combat effectively the pests and mildews that most commonly attack their crop.